1. Field of the Invention
The present invention relates to the structure of a heat radiator of a semiconductor device in which a metal heat radiator is directly or indirectly attached to one of the main surfaces of the semiconductor element.
2. Description of the Prior Art
It is customary in a semiconductor device having a small current capacity that a metal plate supporting the semiconductor element is used as a heat radiator. The semiconductor element is a diode pellet, a transistor pellet or a thyristor pellet.
In general, the semiconductor element is directly coupled to such a metal heat-radiating plate and then subjected to sealing process, or it is hermetically contained in a casing which is fixed to the metal heat-radiating plate.
A semiconductor device for treating current greater than a certain level has an electrode plate interposed between a semiconductor element and a metal heat-radiating plate and the electrode plate has a thermal expansion coefficient approximately equal to that of the semiconductor element, so as to prevent the break of the semiconductor element due to differences in thermal expansion. The semiconductor element is soldered to the electrode plate and the electrode plate is in turn soldered to the metal heat-radiating plate.
When a casing is used, the electrode plate is interposed between the ssemiconductor element and the casing.
The metal heat-radiating plate is used as one of the electrodes of the semiconductor element and must have a good electrical conductivity. It is a matter of course that the heat radiator plate is necessarily made of material having a high thermal conductivity for effectively dissipating heat. Moreover, the material should have a mechanical strength greater than a certain level and a good workability.
A conventional metal heat-radiating plate (heat sink) has been made of a single metal material, e.g. copper or iron.
A copper heat-radiating plate has a good heat conductivity (quality of heat dissipation) and workability, but it also has drawbacks of inferior mechanical strength and very high production cost. On the other hand, an iron heat-radiating plate has a sufficient mechanical strength and is advantageous in cost, but it has a poor heat conductivity. Thus, such a heat-radiating plate of a single metal material has drawbacks as well as advantages.
Especially, in case of a semiconductor device having a rather large current capacity and frequently undergoing heat cycling and mechanical vibration, e.g. a semiconductor full-wave rectifier used with a three-phase alternator in an automobile, the imperfectly designed matal radiator will cause, for example, thermal fatigue in the soldering material, which leads to the destruction of the semiconductor element.